Before Isaac Newton, it was believed that white light was colorless,
and that the prism itself produced the color. Newton's experiments
demonstrated that all the colors already existed in the light in a
heterogeneous fashion

In 1000 CE, Al-Haytham discovered that white light consists of various
rays of coloured light. In the late 13 th and early 14 th centuries,
Qutb al-Din al-Shirazi (1236-1311 CE) and his student Kamal al-Din
al-Farisi (1260- 1320 CE) continued the work of Ibn al- Haytham, and
they were the first to give the correct explanations for the
phenomenon of rainbow. Al-Farisi published his findings in his Kitab
Tanqih al-Manazir [The Revision of (Ibn al-Haytham’s ) Optics]. Newton
did make original discoveries, but this was not one of them.

Did Newton, or Al-Haytham, discover that white light consists of various rays of coloured light?

Many key ideas in science are discovered independently by several people. The real issue is whether Newton could have known of the previous discoveries; if he didn't then he still deserves some credit.
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matt_blackDec 27 '12 at 14:00

2

Newton created a set of (relatively) easily reproducible experiments to prove and measure the light. I think it is probably a bit of an overstatement to think that the belief was that the prism created the light. I suspect that there were other theories about the truth of light that newton set out to prove. The original theory is irrelevant if you can not prove the theory.
–
ChadDec 27 '12 at 14:16

1

@Carlo_R. - Since you just added a bounty to this question, can you specify what details you'd be looking for that aren't already in the fairly extensive answer?
–
BobsonAug 27 '14 at 21:16

2

@Bobson Sorry, I checked the wrong button. I wanted to check 'One or more of the answers is exemplary and worthy of an additional bounty', instead.
–
Carlo AlteregoAug 28 '14 at 6:38

Known in europe as alhazen, this
eminent mathematician, astronomer and physicist,
who came to the fore with a new optics arrived
at by systematic experiments, developed his own
meteorological−optical explanation for the phenomenon of the rainbow in his treatises on the circular
burning glasses and on the rainbow and the halo.
although with his explanation of the origin of the
rainbow through reflection on a concave spherical
cloud, Ibn al−Haitam did not understand the true
state of affairs, nevertheless, he laid a solid foundation for further experiments which, after about 20
years, led to a revolutionary breakthrough.
It was Kamaladdin Abu l−Hasan Muhammad b.
al−Hasan al−Farisi (d. 718/1318), a versatile natural scientist, who declared the explanation of the
preceding scholars about the origin of the rainbow
through simple reflection of light on drops of water
to be incorrect.

A step that Newton took, which (so far as I know) Al-Haytham did not was

Newton set up a prism near his window, and projected a beautiful spectrum 22 feet onto the far wall. Further, to prove that the prism was not coloring the light, he refracted the light back together.

[Newton] gave a detailed explanation of his discoveries in public lectures between 1669 and 1671 (published in 1728 as Lectiones Opticae) and in a paper to the Royal Society in 1672. In 1675, he presented another paper which described further experiments on the colour of thin films and plates, and which put forward a corpuscular theory of light which was surprisingly similar to the modern theory of light quanta. Newton’s Opticks, first published in 1704, went through many editions and was the most influential work on experimental science for almost all of the century. Amongst other things, it explained how raindrops refract sunlight to form rainbows. This was the first chromatic explanation of a phenomenon that had fascinated scientific writers, including Aristotle, Alhazen, Vitello and Antonio de Dominis, since the account of Noah’s Ark was first written down.

Conclusion

It was most likely Newton who first demonstrated that white light consisted of various rays of coloured light. He demonstrated that white light could be split apart into coloured light which could then be recombined to re-form white light thus proving that white light consisted of a mixture of coloured light. I've not found any evidence that anyone prior to Newton was reported as having reached this conclusion and provided convincing proof. There is some evidence that Al-Haytham did not.

From the works he published and from the references to him by later scholars and scientists it is evident that Al-Haytham was a fine scholar and scientist. There is no need to attribute to him discoveries that he did not make.

Newton famously said "If I have seen further it is by standing on the shoulders of giants."